1. Field of the Invention
The present invention relates to a semiconductor device having a function of detecting overheat.
2. Description of the Related Art
Heat is generated in semiconductor integrated circuits due to, for example, an operation of an active element and charges flowing into the semiconductor integrated circuit from outside thereof. Thus, a heat sensing element is formed, and the semiconductor integrated circuit is controlled based on a signal from the heat sensing element, to thereby prevent abnormal operation and breakdown of the circuit caused by overheat. As the heat sensing element, for example, a forward voltage of a PN junction is used. To be specific, when a constant current is caused to flow through a PN junction in a forward direction, potentials of both ends of the PN junction differ from each other. This potential difference is changed depending on temperature, and hence this potential difference is used as a signal for overheating detection (see, for example, Japanese Patent Application Laid-open No. H08-236709 and Japanese Patent Application Laid-open No. H03-034360).
In the field of semiconductor integrated circuits, downsizing of components has been promoted. Because a temperature increase of a component is increased in inverse proportion to an area thereof, a local temperature increase has become intense along with the promotion of downsizing in recent years. As an amount of locally generated heat is increased, a temperature difference between a heat generating source and a heat sensing element is increased. In order to solve this problem, forming the heat generating source and the heat sensing element as close as possible to each other is effective.
In each of Japanese Patent Application Laid-open No. H08-236709 and Japanese Patent Application Laid-open No. H03-034360, a method is employed in which a semiconductor layer 1 that is a heat generating source and a semiconductor layer 2 that is a heat sensing element are isolated from each other by an insulating film, thereby enabling the heat generating source and the heat sensing element to be closely arranged.
In general, the area of the heat generating source is larger than the area of the heat sensing element. Thus, as illustrated in FIG. 1 of Japanese Patent Application Laid-open No. H08-236709, it is preferred that part of the heat generating source be cut out in a plan view so that the heat sensing element can be arranged in the cut out portion. This is because, with this configuration, the heat sensing element is surrounded by the heat generating source, and can thus more accurately sense a temperature of the heat generating source.
In each of Japanese Patent Application Laid-open No. H08-236709 and Japanese Patent Application Laid-open No. H03-034360, a case is specifically described where a power element is formed of vertical transistors. A basic cell of the vertical transistor has a square or an almost square shape in general, and hence it is easy to cut out part of the heat generating source in a plan view so that the heat sensing element can be arranged in the cut out portion, as illustrated in FIG. 1 of Japanese Patent Application Laid-open No. H08-236709.
However, in a case of a power element formed of lateral transistors, part of a heat generating source has not hitherto been cut out in a plan view because of the following difficulty. In this case, the lateral transistors are transistors that have a long width and are arrayed at equal pitches. Further, one common source serves as sources of channels of two transistors. Further, one common drain serves as drains of channels of two transistors. That is, adjacent two transistors have the common source and the common drain, and hence it is difficult to change widths of part of the transistors.
Alternatively, the sources or the drains as described above are not used in common purposely so that widths of part of the transistors can be narrowed, to thereby realize a shape having a portion cut out in a plan view. However, this method has disadvantages in that pitches at which the transistors are arranged are increased and an amount of heat generated per unit area is thus decreased, with the result that a temperature near a heat sensing element is lowered.